This invention relates to ballast devices for igniting and stablizing the operation of two or more electric discharge lamps, and more particularly to an improved hybrid ballast apparatus which provides high efficiency and superior protection against the hazard of electrical shock.
One prior art hybrid ballast apparatus is described in U.S. Pat. No. 4,253,043 issued Feb. 24, 1981 in the name of Chermin et al. The basic Chermin et al system comprises two series-connected discharge tubes stabilized by means of a relatively small ballast. The two series-connected tubes are shunted by a single semiconductor switching element with a bidirectional thyristor characteristic (e.g. a Triac). The control circuit of the semiconductor switching element includes a non-linear voltage dependent circuit element (e.g. a VDR) which is operative to prevent ignition of the discharge tube before the tube filaments heat up. A limitation of the Chermin et al system is that it does not comply with the pertinent safety requirements as to electric shock hazard to persons contacting one pair of tube electrodes while the other pair of electrodes are still in the tube socket.
An improvement over the ballast apparatus shown in the Chermin et al patent is described in U.S. Pat. No. 4,406,976 issued Sept. 27, 1983 in the names of Wisbey and Droho. This patent discloses a hybrid ballast circuit that employs a non-linear voltage dependent resistor (e.g. a Varistor) connected across the ballast capacitor so as to limit the capacitor peak voltage to a predetermined level. This feature thereby limits the maximum voltage appearing between any lamp electrode and ground to a safe value within the specified safety limits. However, although the Wisbey-Droho apparatus was effective to limit the lamp voltages (socket-to-ground) to acceptable safe levels, starting of the lamps in a two lamp series-connected arrangement energized from a 120 volt, 60 Hz AC supply source was not always satisfactory.